Farewell to a tree I loved to hate

I'll be presenting a free Green Newton lecture on Beautiful Sustainable Gardens at the Newton Free Library this Monday June 10 at 7:00 p.m. See you there! 

In April, I received a letter informing me that one of our two street trees was going to be cut down. I’ve been wanting to get rid of this tree for years. It was a Norway maple (Acer platanoides) that had been losing branches over the years, until all that was left was a large limb angling toward the house. 

 

The doomed Norway maple in May

     
     We’re on the side of the street with the power lines, so over the years the utility company has been hacking away at this tree and its partner at the other end of our street frontage. A few years ago, I asked the city to inspect the tree because I thought it was liable to drop a heavy branch on a pedestrian or a parked car. At that time, they found it was still sound. Now, the letter said, it had “significant defects.”
 

Marked to be cut down

     The reason I resented this tree wasn’t just its species. Yes, I dislike Norway maples for their fecundity and voracious shallow roots. This tree also cast dense shade over the front yard. It had limited what I could grow under its branches to the standard broadleaf evergreens you see everywhere in the neighborhood. A couple of years ago I added an oakleaf hydrangea (Hydrangea quercifolia) that livened things up a little, but still.

Ho hum boxwood, Japanese pieris, and mountain laurel

    One morning last week, two trucks arrived, and the urban forestry team took down the tree. I darted in and out of the house taking pictures to document the big event. 

Avoiding wires to saw off chunks

A bucket truck allowed a man with a chain saw to maneuver around the wires and cut the tree apart piece by piece. Another man operated a giant arm picking up large branches to haul back to the city yard for chipping. 

Future wood chip mulch

After half an hour, all that was left was the newly cut stump.

Its center was hollow

    I had six weeks to anticipate it, but this event caught me off balance. I wasn’t prepared to be sad to see the Norway maple go. Although I’d wanted it gone, seeing it chopped up so fast reminded me of how it had stood there patiently through blizzards, hurricanes, droughts, and nonnative insect attacks. The tree was misshapen at the end, but that comes to all who live to old age.

    The other surprise was my lack of ideas for what to add to that corner of the front yard. Without the tree’s shade, there are so many more options, including native shrubs I’d like to try. I’m not used to selecting plants for sunny spots. I’ve often thought that more highbush blueberries (Vaccinium corymbosum) should be grown in front yards. They have small neat leaves like privet (Ligustrum vulgare) that turn a pretty red in fall. 

Blueberry foliage turns red in fall

Unlike privet, though, they’re natives and not invasive. It would be a friendly gesture toward both birds and walkers to offer blueberries along the sidewalk.

Neighbors could pick blueberries as they passed

    When it’s too hot to work outside this summer, I’ll consider what else I could plant in the new sunny clearing this fall.

Good news for trees

Spotting a little green caterpillar on a weed I pulled from the perennial bed reminded me that this is the time when winter moth larvae (Operophtera brumata) make their way up trunks of trees such as the old oak that stands over this area of the garden.

Winter moth caterpillar

    That caterpillar was in for a surprise, though. Thanks to a team led by University of Massachusetts entomologist Joseph Elkinton, a parasitoid tachinid fly, Cyzenis albicans, is laying its eggs on leaves consumed by winter moth caterpillars. The fly’s offspring will kill most of the caterpillars that eat those leaves. What used to be an epidemic of nonnative pests defoliating trees in eastern Massachusetts has turned into no big deal.

    We used to notice clouds of male winter moths when they appeared in late fall, often lighting on windows and doors and sometimes making their way into the house. 

Winter moth caterpillars in November 2016

The wingless females can’t fly, but they can climb tree trunks to lay their eggs in bark crevices. The larvae emerge in March and devour leaf and flower buds, or they produce strands of silk that allow them to “balloon,” floating to other trees to spread the destruction. 

     Winter moth surged in Massachusetts in 2003 and found no native predators. At the height of the moths’ population boom, they affected most of our street trees, favoring the plentiful maples and oaks. It was alarming to walk down the street and see every tree leaf riddled with holes.

Birch leaf chewed by winter moth caterpillars, 2015

    To protect ornamental trees, the usual approach was to apply pesticides, and I did that for several years. I was telling myself that the treatment was relatively benign, because it used spinosad, an agent derived from naturally occurring soil bacteria.

Although it's a natural product, spinosad kills bees as well as winter moths

     Elkinton’s team took a different tack. In Europe, where it originated, winter moth has numerous insect predators. The scientists tested these to see which would be safe for release in New England, seeking an insect that targeted only winter moth and wouldn’t harm native insects. For the past 14 years, they’ve been releasing the parasitoid fly Cyzenis albicans to kill winter moth caterpillars. 

Cyzenis albicans-photo James K. Lindsey

    This year Elkinton has declared victory. After years of painstaking monitoring, his team has determined that winter moth populations are down to manageable levels. The parasitoid fly has become established, maintaining sufficient populations to keep winter moth under control.

    Predators need prey. If the flies eradicated all the winter moths, they’d have no food for their larvae. They would die out too. With both maintaining a stable presence, they can continue their interdependent dance.

    This is one of those scientific and ethical dilemmas where human beings have created a problem—in this case by unintentionally importing nonnative winter moths—and there’s no perfect response. If we did nothing, we’d likely lose many trees. 
  

     If we kept spraying pesticides, even targeted ones, we’d inevitably cause collateral damage to other insect species. That's why I stopped applying spinosad. So far, introducing nonnative Cyzenis albicans seems like the happiest solution for a thorny environmental problem.

Winter moths won't be troubling our oak anymore

What goes with chartreuse?

After the beautiful elms that lined our streets succumbed to Dutch elm disease in the mid-twentieth century, my town, like many others, thought they’d found great street trees to replace them: Norway maples (Acer platanoides). 

Surviving elms in New York's Central Park

After all, the maples were tough, adaptable growers that could survive the tight root space, thin, rocky topsoil, and plentiful road salt that our narrow curb strips offered.
    
    In one way, choosing these maples was a success. Almost all our streets are now lined with mature specimens. Over their lifetime they’ve rolled with the punches, putting up with limbs torn off by ice storms and passing trucks, finding water by pushing their roots under the sidewalks into lawns and garden beds, growing around holes gouged in their canopy by pruning around power lines.

Norway maple street tree-photo Famartin

    The problem with these nonnative trees is that they’re too successful. Each tree produces thousands of viable seeds every spring, packing pairs of them into the helicopter-like samaras that we see twirling to the ground. 

Winged seed pods of Norway maples

They lie in cracks in the sidewalk, in garden beds, and on any bare soil or pile of organic debris. It seems as if most of those seeds succeed in producing seedlings the following spring. 

    This is the time of year when gardeners like me become obsessed with weeding out those seedlings. We know that by next year, this season’s tender little sprout will have grown a woody stem and sent down a deep root. If we don’t catch them this year, they’ll be harder to pull with every passing year.

One-year-old Norway maple seedling, left, with this year's on right

    When we moved into our house, a dense thicket of Norway maples that soon reached 20 feet high grew behind the back fence, just because our neighbors had left some seedlings alone to grow. The trees make dense shade, their shallow roots crowd out competitors, and they exude allelopathic chemicals that keep other plants from thriving in their vicinity.

Norway maples shading the backyard in the mid-90s

Luckily for my garden, we were able to remove those trees. Their relatives have continued to colonize the city, though. 

    When the Norway maples flower at the end of April, my heart softens toward them, despite my struggle to curb their reproduction. I love the lacy silhouettes the flowers make against the street lights after dark. The rosettes of chartreuse flowers and tiny new leaves are a heartening sight when many trees haven’t yet put out any foliage. 

Norway maple in bloom

A cloud of yellow-green covers the whole city, reminding me of Robert Frost’s line, “Nature’s first green is gold.”

Norway maples paint the landscape a vibrant pale green

    In recent years, our city has gotten wise. Where street trees have died or been cut down, our urban forestry director now chooses to plant a range of resilient tree species—but no more Norway maples. Soon we’ll have a diverse mix of street trees that are less inclined toward world domination. Meanwhile, we should remind ourselves when choosing spring-blooming bulbs and shrubs that they’re going to flower against a chartreuse background. Whether we like it or not, those maples won’t disappear anytime soon.